A photonic integrated circuit (PIC) is an integrated optical system that provides the generation and manipulation of light-based signals on a single substrate. A conventional PIC typically includes active optical devices (e.g., lasers, receivers, semiconductor optical amplifiers (SOA), detectors etc.), and planar lightwave circuits (PLC) comprising one or more surface waveguides arranged to provide passive optical functionality (e.g., waveguiding, power splitting, wavelength modulation and demodulation, power combining, switching, etc.). PICs are seen as key enablers in many important applications, including optical telecom and datacom, sensors, light projection, high performance computing, space-based communications, and the like.
One of the most common PIC configurations is that of photonic devices and/or photonic integrated circuits disposed on silicon-based substrates (often referred to as “silicon photonics”). Silicon photonics leverages the well-established large-area processing capabilities of CMOS integrated circuitry, thereby promising low-cost, high-volume solutions. In addition, the use of a silicon substrate enables the addition of microelectronic circuitry to a PIC, thereby enabling augmentation of the optical functionality of a PIC with complex electrical functionality.
Unfortunately, the development of such PIC configurations is complicated by the fact that silicon has an indirect bandgap; therefore, it is not well suited for use in the active optical devices needed for PICs to fully function. As a result, current implementations of silicon-based PICs rely on the use of compound-semiconductor-based photonic devices (e.g., lasers, modulators, semiconductor optical amplifiers, etc.) that are wafer bonded (heterogeneous integration) or coupled externally (hybrid integration) to a silicon substrate comprising the passive optical elements, such as one or more PLCs and/or silicon-based photonic elements (e.g., detectors, etc.). Such approaches require the use of expensive compound-semiconductor native substrates on which the photonic devices are grown via epitaxial growth processes.
To date, the commercial success of heterogeneous- and hybrid-integrated PICs has been limited by the high cost of compound-semiconductor substrates as compared to silicon-based substrates, as well as the cost and complexity of the extra processing steps associated with the integration of the active devices with the silicon-based passive optical devices.
Epitaxial growth of compound-semiconductor materials directly on a silicon substrate (i.e., “hetero-epitaxial growth”) has been pursued in the prior-art as an alternative to heterogeneous- and hybrid-integration. It is particularly attractive because the realization of epitaxially integrated active photonic devices with silicon-based substrates would enable both cost reduction as well as performance improvements for commercial PICs, since it would eliminate the need for the smaller and relatively more expensive native compound-semiconductor substrates. Furthermore, it enables improved scalability. The scalability of direct epitaxial growth on silicon-based substrates is limited to the size of the largest available silicon-based substrates, while the scalability of heterogeneous- and hybrid-integration approaches is limited by the size of the commercially available native compound-semiconductor substrates on which the active optical devices are grown. The largest silicon-based substrates available today are twice the diameter of the largest compound-semiconductor substrates.
Unfortunately, good hetero-epitaxially grown active optical devices are difficult to achieve. Hetero-epitaxially grown material is known to have notoriously high defect densities, which arise due to the mismatch between the lattice constants of the grown material and the substrate on which it is grown. As the thickness of the hetero-epitaxially grown layer grows beyond a “critical thickness” inversely proportional to the mismatch in lattice constants and typically on the order of a few nanometers (required to support quantum wells and optical gain, for example), threading dislocations form in the material, thereby compromising its quality. As a result, lasers grown hetero-epitaxially on silicon tend to degrade rapidly and have extremely short lifetimes. It is unlikely, therefore, that prior-art hetero-epitaxial-growth approaches to forming PICs will ultimately result in the rapid design and deployment of the broad range of low-cost components demanded by the rapidly expanding application space.
As a result, there remains a need for a simple, low-cost approach to integrating direct-bandgap-material-based devices and indirect-bandgap-material-based substrates to form practical, commercially viable photonic integrated circuits.